1. Field of the Invention
The present invention relates to a method and apparatus for irradiating laser and, more particularly, to the technique such as used in a laser annealing process performed for forming a polycrystalline thin film by irradiating a semiconductor thin film with a laser.
2. Description of the Related Art
A polycrystalline silicon thin film (referred to as polycrystalline-Si film hereinafter) is adopted as a conductive film of a thin-film-transistor (TFT) device formed on a glass substrate, because of the easiness for processing. Generally, an excimer laser annealing (ELA) technique is employed to form the polycrystalline-Si film. In the ELA process, an amorphous silicon thin film (referred to as amorphous-Si film hereianafter) is formed on the substrate, and a pulse beam of excimer laser is irradiated on the thus formed amorphous-Si film. The amorphous-Si film is melted by the irradiation of the laser, and then recrystallized through a cooling down, consequently to form a polycrystalline-Si film. It is generally known that a larger grain size of the polycrystalline-Si film provides excellent properties for the TFT device, especially in the carrier mobility thereof. Therefore, it is desired to form the crystal grain as large as possible in the polycrystallization of the amorphous-Si film.
In the process of polycrystallizing the amorphous-Si film, along with an increase in the irradiation intensity of laser, a shift is caused from a crystallization (ordinary crystallization) wherein a relatively large crystal grain is obtained toward a microcrystallization wherein a finer crystal grain is obtained. The ordinary crystallization is caused when the amorphous-Si film is not completely melted, and thus nuclei are generated at random at the boundary surface between the solid phase and the liquid phase of silicon or the boundary surface between the amorphous-Si film and the substrate, during the recrystallization. In the ordinary crystallization, the diameter of the crystal grain increases as the irradiation intensity is increased.
On the other hand, the microcrystallization is caused when the amorphous-Si film is completely melted, and thus nuclei are generated in a relatively uniform state in the entire film, during the recrystallization. The diameter of the crystal grain created in the microcrystallization is as small as 100 nm or below. In the microcrystallization, the diameter of the crystal grain scarcely depends on the irradiation intensity. The threshold intensity of the laser at which the shift from the ordinary crystallization toward the microcrystallization occurs is referred to as “microcrystallization intensity”. The microcrystallization intensity is a parameter normalized by the thickness of the amorphous-Si film in general.
As described above, in the process of polycrystallizing the amorphous-Si film, the shift from the ordinary crystallization toward the microcrystallization, bounded by the microcrystallization intensity, is abruptly caused with an increase in the irradiation intensity of the laser. That is, when the irradiation intensity of the laser exceeds the microcrystallization intensity, the grain size of the resultant polycrystalline-Si film extremely reduces, whereby a suitable grain size is not achieved. Therefore, in the process of polycrystallizing the amorphous-Si film, it is important to accurately determine the microcrystallization intensity and set the irradiation intensity of the laser smaller than the microcrystallization intensity.
Patent Publications JP-2000-114174A and JP-2002-8976A describe a technique for determining the microcrystallization intensity. According to these patent publications, prior to actual irradiation for forming a polycrystalline-Si film in a TFT device, preliminary irradiation is performed in which the laser is irradiated on a pulse-by-pulse basis onto preliminary irradiation areas provided outside a target irradiation area, on which the TFT device is to be formed. The preliminary irradiation is performed while moving the irradiation position and changing the irradiation intensity within a range over and below the melting intensity of the amorphous-Si film. The relationship between the irradiation intensity and the presence/absence of microcrystallization is derived from the spectral measurement of a Raman light or scattered light for the preliminary irradiation areas, and the radiation intensity at which the microcrystallization starts is determined to be the microcrystallization intensity.
According to the technique of the above patent publications, the actual irradiation is performed by irradiating the laser at the intensity lower than the microcrystallization intensity thus determined, which allows the microcrystallization of the amorphous-Si film to be prevented. In the technique of these documents, however, the spectral measurement of a Raman light or scattered light must be performed while the light source for measurement and the light-receiving position for the reflected light are moved to respective preliminary irradiation areas. This impedes improvement in throughput of forming the polycrystalline-Si film.